For mobile communication applications with high demands on Radio-Frequency (RF) filter selectivity, thin-film Bulk Acoustic Wave (BAW) filters are the technology of choice. Examples of this trend include narrow band CDMA and WCDMA mobile devices equipped with BAW filters. Such filters are particularly suitable for communication standards with carrier frequencies above 1 GHz and having stringent specifications on frequency selectivity.
An additional desirable feature for a BAW device is transformation of an unbalanced input signal into two balanced output signals. This action is called “balun” (balanced-to-unbalanced or vice versa) functionality. If a circuit utilises conventional BAW devices for filtering, then a separate balun component is required. Balun functionality was, in the past, traditionally achieved by magnetic coupling—using a transformer. As is well known, the DC levels at either side of a transformer can be chosen independently. It is desirable to provide similar decoupling using acoustic coupling between BAW devices, rather than magnetic coupling between transformer coils.
One proposal to achieve integrated balun functionality is the acoustically-coupled BAW. Two variations of this idea have been proposed: vertically and laterally coupled BAWs. The vertical version (hereinafter VBAW) is difficult and expensive to produce. On the other hand, laterally coupled BAWs (hereinafter LBAWs) are potentially cheap, but good performance devices have not been reported: neither good frequency selectivity, nor balun functionality have been shown for LBAWs.
Even without the added feature of balun functionality, acoustically coupled BAW filters have benefits for design flexibility: by adding shunt capacitances, the designer has full control of the notches outside the pass-band, whereas in conventional BAW filters they are fixed by the device technology.
It is therefore desired to produce a BAW device—and in particular a coupled BAW device—having good frequency selectivity properties, which is also easy and inexpensive to manufacture.